1. Field of the Invention
The present invention relates to an copper alloy sheet used for electric and electronic parts, e.g., terminal-connectors and relays, materials for semiconductors (lead frames, heat sinks), materials for electric circuits (automobile junction blocks, consumer electric part circuits), and the like.
2. Description of the Related Art
In an automobile field, many electric and electronic parts have become mounted in order to comply with environmental regulation and pursuit the comfortableness and the safety, and narrowing pitches and miniaturization have been required of terminal-connectors, relays, and the like employed. In addition, the same have been required in information communications and consumer-oriented fields. Consequently, materials having higher 0.2% proof stress, electrical conductivity, bending workability, stress relaxation resistance have been required as electric and electronic part copper alloy sheets.
The 0.2% proof stress refers to a force required for inducing 0.2% of plastic deformation of the material. If the 0.2% proof stress is high, it is possible to keep contact while a strong force is applied to a contact point. Furthermore, the same contact pressure can be obtained by a small sheet width or a thin sheet.
The electrical conductivity refers to ease of passing of electricity and is represented by a ratio (% IACS), where the electrical conductivity of pure copper (IACS) is specified to be 100%. The electrical conductivity and the volume resistivity (μΩ·cm) are in inverse proportion. In copper alloy whose electrical conductivity is high, the volume resistivity is low, and Joule heat generation can be suppressed.
The bending workability is evaluated by the ratio (R/t) of the minimum bend radius R, at which cracking does not occur, to the sheet thickness t. A material having good bending workability contributes to stabilization of the quality and, in addition, improves the design flexibility in pressing. Severe bending has been performed with a bend line perpendicular to a rolling direction (G.W) previously. However, cases in which bending is performed with a bend line parallel to a rolling direction (B.W.) have increased because of diversification in design technique.
The stress relaxation resistance refers to the durability to a phenomenon in which the contact pressure is reduced with time under a high-temperature environment, that is, stress relaxation. The stress relaxation resistance is indicated by a stress relaxation ratio or a residual stress expressed in “%” after holding under predetermined load stress, temperature, and time conditions. A material having good stress relaxation resistance can be used, for example, in the vicinity of an automobile engine room and, therefore, contributes to improvements in design flexibility and reliability of electric equipment to a great extent.
A Cu—Ni—Si base copper alloy has all of these characteristics and is widely used as electric and electronic part copper alloy sheets at present. The Cu—Ni—Si base alloy is an alloy having increased 0.2% proof stress and electrical conductivity by aging precipitation of a Ni—Si compound from a supersaturated solid solution. In the case where the Cu—Ni—Si base alloy is subjected to a high-temperature short-time heat treatment referred to as a solution treatment, a recrystallized grain structure can be formed. The bending workability of a material having a recrystallized grain structure is considerably improved as compared with the bending workability of a material having a worked structure.
In addition, the Cu—Ni—Si base alloy is a precipitation strengthening alloy and, therefore, can obtain a high 0.2% proof stress while the working strain is kept at a low level as compared with a solution strengthening alloy in the related art. If much working strain is accumulated, dislocation in a material structure is relaxed easily, and the stress relaxation resistance is degraded. That is, the Cu—Ni—Si base alloy is superior to other alloy systems in the stress relaxation resistance as well.
Meanwhile, there is a so-called trade-off relationship in which if any one of the characteristics of the 0.2% proof stress, the electrical conductivity, the bending workability, and the stress relaxation resistance of the above-described Cu—Ni—Si base alloy is further improved, the other characteristics are degraded. Consequently, in many cases, improvements of the characteristics are prevented. It is particularly difficult to ensure compatibility between the bending workability and the stress relaxation resistance because the bending workability becomes good when the crystal grain size is small and the stress relaxation resistance becomes good when the crystal grain size is large. Therefore, there is a previously proposed technique in which the bending workability is improved mainly by controlling the crystal grain size and the stress relaxation resistance is improved mainly by adding an element or elements.
In Japanese Patent Application Publication No. 2008-75152, Japanese Patent Application Publication No. 2008-196042, Japanese Patent Application Publication No. 2008-266783, Japanese Patent Application Publication No. 2007-146293, and Japanese Patent Application Publication No. 11-335756 disclose methods for improving the bending workability or the stress relaxation resistance of Cu—Ni—Si base copper alloys. Among them, Japanese Patent Application Publication No. 2008-75152, Japanese Patent Application Publication No. 2008-196042, and Japanese Patent Application Publication No. 2008-266783 disclose methods for improving the bending workability of the Cu—Ni—Si base copper alloys by controlling the crystal grain sizes. Japanese Patent Application Publication No. 2007-146293 discloses a method for improving the stress relaxation resistance of the Cu—Ni—Si base copper alloy by controlling additional elements. Unexamined Patent Application Publication No. 11-335756 discloses a method for improving the stress relaxation resistance by controlling additional elements and improving the bending workability by controlling the crystal grain size.
As shown in the above-described five patent literatures, the bending workability of the Cu—Ni—Si base alloy has been improved mainly by controlling the crystal grain size and the stress relaxation resistance has been improved mainly by controlling the addition of elements. However, there are problems in that an improvement in bending workability by control of the crystal grain size, specifically, reduction in the crystal grain size, is accompanied by degradation in stress relaxation resistance, and an improvement in stress relaxation resistance by the additional elements is accompanied by degradation in electrical conductivity and bending workability, although not described in the above-described five patent literatures. In addition, in order to obtain predetermined bending workability and stress relaxation resistance, it is required that the crystal grain size falls within a predetermined range through the recrystallization by a solution treatment. However, there is a problem in that crystal grains become coarse sharply in accordance with changes in treatment temperature depending on a desired crystal grain size and variations occur in the characteristics of a product.